I found some more info on-line. Grillecloth had a vague reference in one of their archives, and showed the correct lineup. I then searched and found Mallory repair guides online (I'll post the link when II find it again). Of those, one included the 388 set, indicated it has a 460 kHz I.F. and also control circuit and part details.

Back in the UK, I have a Riders manual. We're heading there next week so I'll pull it out and try some more detective work. It would be good to have more circuit info.